The part that I mind about both delayed_paperclip and carrierwave_backgrounder is that it doesn't really allow you to write your own job classes. This can be quite limiting for example if you want your jobs to send information to some monitoring services.
Another part that I mind is the user experience. Ok, both of these gems allow you to know when the backgrond job is in process, and then you can display some placeholder image until the processing is done. But that's not really nice user experience. In Shrine the user immediately sees the image they uploaded, because it has that image already cached (usually on the filesystem), and then from the user's point of view the uploading is finished, before the background job even started. CarrierWave also has the cached image, but by design it's not possible to use it, while Paperclip doesn't cache images.